How does electronic copyright work?

I’m a one-man business mainly creating design for print and handling the print on most occasions. From time to time clients will ask for a particular job to be downloaded on to disk so that either they, or one of their other suppliers, can make alterations and have it either reprinted or used for the Web or to be used in some other format.

Should I be making a charge for this further service, and if so how much? Perhaps a percentage of the original job? And how would you describe this service?

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